1. Field of the Invention
The present invention relates to a secondary air supply system for supplying secondary air into the exhaust system of internal combustion engines for the purpose of purifying exhaust gases.
2. Description of the Prior Art
It is known to employ a secondary air supply system as a countermeasure for purifying exhaust gases from internal combustion engines so that harmful uncombusted components such as HC and CO contained in the exhaust gases are further combusted in the exhaust system under a supply of secondary air. In order to obtain the highest efficiency of exhaust gas purification and from the point of view of protecting various devices included in the exhaust system from being damaged, it is favorable that the amount of secondary air introduced into the exhaust system is always proportional to the amount of intake air of the engine, i.e., the ratio of secondary air flow to intake air flow is maintained at a constant value. However, such a manner of control for the supply of secondary air is not effected in conventional secondary air supply systems.
On the other hand, the regulations with regard to exhaust gas contamination are becoming very strict and it is required that the efficiency of exhaust gas purification be improved.